Will the irs reject me efile if someone unlawfully claimed my children on their return? HR Block said wait another 21 days an then re efile so I did.

If you mailed your return then do not efile it again.   You can only file 1 time.   Mailing it was the right thing to do.  

Yes, your return will be rejected unless you have an IRS IP PIN - See IRS News Release   Without the IP PIN, you are required to file the return by mail.   The IRS will intervene when more than one taxpayer claims a dependent(s).    See this TurboTax Article on this issue.